Who needs sex? Six animals that cloned alone

In 2001, the aquarists at Omaha's Henry Doorly Zoo and Aquarium in Nebraska were astonished to find this baby bonnethead shark (Sphyrna tiburo) swimming around in one of their tanks. The three female bonnetheads in the tank had been caught when very young and had been there for three years without contact with a male – so how could the pup have been conceived?

Various explanations were proposed: perhaps one of the sharks had retained sperm from a mating before being caught, for instance. In 2007, genetic tests ended such speculation. The pup was the product of a "virgin birth", or parthenogenesis – in other words, a clone of her mother. She was unfortunately long dead by then, killed by a stingray days after her birth.

(Image: Omaha's Henry Doorly Zoo)

A baby Komodo dragon (Varanus komodoensis) pokes out of his egg at Chester Zoo, UK, in January 2007. His was one of a clutch of eggs laid by the zoo's female dragon Flora, which had never mated with, or even met, a male dragon. Tests carried out on the eggs the preceding year confirmed that they were the result of parthenogenesis.

In Komodo dragons, parthenogenesis produces only male offspring. This is because Komodo dragons, among some other reptiles, are unlike mammals in their sexual set-up: it's the females that have a mixture of male and female sex chromosomes. In these animals, parthenogenesis produces embryos with either two female sex chromosomes – which cannot survive – or two male chromosomes.

(Image: Phil Noble/Reuters)

This young male copperhead snake (Agkistrodon contortrix), curled up in the coils of his mother, wasn't supposed to exist. Even as the number of confirmed cases of parthenogenesis in large species grew, many biologists continued to regard it as an anomaly seen only in captivity. But when a team in the US caught wild pregnant females from two species of pit vipers and tested the offspring, they discovered two of the litters were parthenogenetic. So some species may reproduce in the wild either sexually or by parthenogenesis.

"Regarding the baby, it is a male, was born in 2011, and as far as fertile… that's the million-dollar question," says Charles Smith of Wofford College in Spartanburg, South Carolina, a member of the team. "We are raising him up in the lab and will hopefully be able to test him for fertility this summer."

(Image: Charles Smith)

This zebra shark (Stegostoma fasciatum) at the Burj Al Arab aquarium in Dubai starting laying eggs each year from 2007, despite a distinct lack of males. Eggs from lone females are often discarded by aquaria, but the staff who found these eggs, David Robinson and Warren Baverstock, had heard about virgin births in sharks. So they kept them to see what happened and sure enough, some hatched.

(Image: Warren Baverstock)

This caption has been edited since it was first published.

For this blue-spotted salamander (Ambystoma laterale), parthenogenesis is not an option – it's how it reproduces all the time. There are no males of the species. Bizarrely, though, the salamanders still need males, because their eggs have to be "activated" by sperm. So they mate with males of other species, but the all-female offspring do not usually inherit any of the male's DNA. This peculiar arrangement is known as gynogenesis.

Altogether around 90 species of all-female vertebrates have been discovered, but most still mate with males of other species, for reasons that have only recently become clear.

(Joel Sartore/NGS)

In theory, there are lots of advantages to ditching males altogether. There is no need to waste time looking for a mate, for instance, or risking catching STDs. Yet despite this, surprisingly few animals have dispensed with males. The common checkered whiptail lizard, Aspidoscelis tesselata, is one of them, along with a few other species of whiptails and some geckos. These animals do sometimes still show signs of mating behaviour, such as engaging in "pseudocopulation" with other females.